The relationship between central cholinergic neurotransmitter systems and Alzheimer's disease (AD) continues to offer insight into the pathologic basis of this devastating illness. Cholinergic neurons are lost in AD, and pharmacologic therapies designed to augment cholinergic function show some modest clinical efficacy. Cholinergic neurotransmission is mediated in part by a family of five similar but distinct muscarinic acetylcholine receptors (M1-M5 mAChRs). mAChR subtypes appear to differentially regulate the processing of amyloid precursor protein (APP), a pivotal molecule in AD pathobiology that gives rise to the A-beta peptide via proteolytic processing, but the precise contributions of individual mAChR subtypes to the development and progression of AD remains incompletely understood. In this study, we propose to use a combination of genetic, biochemical, and histologic techniques to define the molecular subtypes of muscarinic receptors responsible for modulating AD pathology. Based on previous reports and our own preliminary evidence, our central hypothesis is that the M1 subtype promotes non-amyloidogenic processing of APP and limits disease progression, while M2 and/or M4 subtypes may accelerate the amyloidogenic processing of APP. In Aim 1, we will make use of primary neuronal cultures from mice deficient in mAChR subtypes to evaluate the role of mAChR signaling on APP processing. We hypothesize that loss of M1 receptors will increase amyloidogenic APP processing, while loss of M2 and/or M4 receptors may limit amyloidogenic APP processing. In Aim 2, we will extend the mechanistic findings of Aim 1 into an in vivo model of AD by crossing M1 mAChR knockout (KO) mice with the APPswe/ind transgenic mouse model. We hypothesize that APPswe/ind mice crossed with M1 KO mice will display more severe and accelerated amyloid pathology as compared to M1 (+/+) littermates. Aim 3 will investigate the molecular mechanisms underlying mAChR subtype regulation of APP processing, with the hypothesis that M1 activation induces the activation and translocation of the alpha-secretase candidate ADAM 17. Alzheimer's disease (AD) is the leading cause of dementia in the elderly and as such, represents an immense and continually growing burden on the health and prosperity of our society. By investigating the relationship of muscarinic acetylcholine receptors to the underlying cause of AD, this study seeks to increase the molecular understanding and potential therapeutic avenues of this devastating illness.